DE69714802T2 - Illumination device with improved visibility for a film with medical diagnostic images - Google Patents

Description

Background of the invention

The invention disclosed and claimed herein relates generally to an improved film viewing box such as is used to illuminate a film of X-rays or other medical diagnostic images for viewing and study. More specifically, the invention relates to a device of this type in which the contrast and resolution or sharpness of a viewed image is significantly improved or increased so that visibility is improved.

As is well known, a film viewing box, also called a light box, is a device used by radiologists to view and study x-rays or other medical diagnostic images on film. Such images, which show selected views of body tissue, are obtained using conventional x-rays, computed tomography (CT), magnetic resonance imaging (MRI), and other techniques known to those skilled in the art of medical diagnostic imaging. By carefully studying such images, a radiologist can detect the presence of significant medical conditions in a patient or other subject without the need for invasive surgery. Some important examples of such conditions include breast cancer, lung cancer, pneumonia, fractures, and arthritis.

A typical film viewing box contains one or more fluorescent tubes or other light sources and has a front panel that typically contains a translucent diffuser, such as a sheet of milky white Plexiglas. The film is placed on the light diffuser and illuminated with light from the light source for viewing and study. The light passes through the diffuser and is thereby diffused or scattered. In this way, the light transmitted to the film frame has a uniform brightness or intensity, so that the film is evenly illuminated.

If a film viewing cabinet is not equipped with a diffuser or other means of scattering the light from the fluorescent light source, the fluorescent tubes can be seen through the film, making it difficult to view. However, the scattering or diffusion of the illuminating light degrades the contrast between lighter and darker areas of the film image, as well as the resolution from the viewer's perspective. Because the light that brings the film image to the viewer's eye is scattered, the edges and boundaries between lighter and darker areas of the image become blurred when it reaches the viewer's retina, making the radiologist's job of correctly interpreting a film image more difficult. Certain conditions, such as breast cancer, lung nodules, and small pneumothorax (collapsed lung), are particularly difficult to detect in any case. If the edges are blurred too much or the contrast between lighter and darker areas of the film image is insufficient, accurate diagnosis of such conditions may even become impossible. The term "resolution" as used here means the degree of sharpness or Smearing, which marks the boundary or edge between adjacent lighter and darker image areas.

The importance of high contrast and resolution in the illumination of diagnostic imaging films is further emphasized by certain U.S. government regulations. Federal law requires that all X-ray equipment used in the United States for mammography, i.e., obtaining radiographic images for the detection of breast cancer, be periodically tested for contrast and resolution. Such a test is performed by producing a radiographic image of a standardized mammography phantom, which is analyzed by a government inspector or medical personnel. If the inspector determines that the contrast or resolution of the image is inadequate, the operator of the equipment will be denied permission to perform further mammograms until the situation is corrected. These government regulations have been established because if there is insufficient contrast between two distinct areas of slightly different density in a breast, or if there is insufficient resolution along the edge between them, a small breast cancer, which is characterized by only slight differences in density from normal breast tissue, may not be detected. This in turn may lead to a delay in treatment that can cause great harm to a patient.

In the past, it has been proposed to increase the contrast of a medical diagnostic film image illuminated by a film viewing box by significantly increasing the spatial distance between the light source and the film image. This makes the light illuminating the film less diffuse, ie its solid angle decreases, as will be explained in more detail below. However, the proposed spatial separation was on the order of 1.8 to 3.6 m (six to twelve feet), whereas the depth of a typical film viewing box is generally less than 0.3 m (one foot). It would be highly impractical to build a film viewing box with the increased dimensions proposed for a number of reasons, including cost and space constraints.

The inventor is not aware of any prior art relating to reducing the diffusion of the illumination light to improve the resolution.

An apparatus for illuminating and viewing transparent film images such as x-ray films, mammograms and sonograms is described in US 5,313,726. It has a housing containing a light source at one end and a diffuser and film support at the opposite open end, and an optical element arranged between the light source and the diffuser for reducing optical artifacts caused by a programmable liquid crystal light valve arrangement provided between the light source and the optical element. The optical element may comprise lens arrangements, fiber optic end faces and a series of light guide tubes.

WO 95/34009A describes a flat transparent front lighting device using microprisms. This system is particularly suitable for illuminating objects located directly below the device.

JP 05089827A describes a black light device for a liquid crystal display, which is used for example for an electric viewfinder in a VTR. The device contains a light source, a diffuser plate and a microprism condenser sheet for directing diffused light in a forward direction in order to increase the brightness of the display.

Summary of the invention

The invention provides a device for illuminating a film with images used for medical diagnosis, which includes means for holding the image film in a certain plane. The invention also has means for projecting light onto the held image film, the projected light having a first solid angle. A light-directing film element is positioned between the image film and the light projection device so that the projected light falls on it. This selectively reflects and/or refracts at least a portion of the projected light and reorients the light for illuminating the image film, the illuminating light having a second solid angle that is substantially smaller than the first solid angle. Preferably, the light-directing film element comprises one or more layers of a film or foil, on one side of which a number of microscopic prism elements or microprisms are formed.

The term "solid angle" as used here is a measure of the divergence, diffusion or scattering of a quantity of light, such as the light directed onto or illuminating the plane of the film. The solid angle is oriented along an axis perpendicular to the film plane and can vary from 0º, i.e. parallel light, to the order of 180º, i.e. strongly scattered or diffuse light.

In a preferred embodiment, the light-directing film element is characterized by a critical angle and runs parallel to the specific plane. Some of the rays of the projected light, each directed along lines that intersect the specific plane at angles that are smaller than the critical angle, comprise a first portion of the projected light. Other rays of the projected light, each directed along lines that intersect the specific plane at angles that are larger than the specific critical angle, comprise a second portion of the projected light. The light-directing film element prevents the first portion of the projected light from passing through to the image film and refracts the second portion of the projected light so that corresponding rays from it are converged so that they lie within the second solid angle.

Objectives of the invention

One object of the invention is to improve or increase the contrast and resolution of a device for illuminating radiographic or other images on film for medical diagnosis.

A further objective is to create a film viewing box for diagnostic imaging film in which contrast and resolution are enhanced without a significant increase in cost or complexity compared to known viewing boxes.

A further task is to create a film viewing cabinet of the type mentioned above, which maintains the dimensions of currently used film viewing cabinets.

A further object is to create a film viewing cabinet of the above-mentioned type, which can be easily manufactured by converting a known film viewing cabinet with comparable simplicity.

These and other objects of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings.

Short description of the drawings

Fig. 1 shows a perspective view of a known film viewing box.

Fig. 2 is a set of diagrams illustrating the respective effects of scattered and parallel light in transilluminating film containing images for medical diagnosis.

Fig. 3 is a side view of an embodiment of the invention.

Fig. 4 shows a film material with parallel rows of microprisms which can be usefully used in the embodiment of Fig. 3.

Fig. 5 shows a partial section along the line 5-5 of Fig. 4.

Fig. 6 shows a perspective view of a second embodiment of the invention.

Fig. 7 shows a section along the line 7-7 of Fig. 6.

Fig. 8 shows a section along the line 8-8 of Fig. 6.

Fig. 9 shows a perspective view of a third embodiment of the invention.

Fig. 10 shows a set of diagrams to explain the effect of the invention in reducing the solid angle of the light illuminating the film as compared to light which is randomly scattered.

Fig. 11 and 12 together show an arrangement used for comparing the contrast resulting from the embodiment of Fig. 9, as well as a film viewing box according to the prior art.

Detailed description of the preferred embodiment

Referring to Fig. 1, there is shown a conventional film viewing cabinet 10 which generally includes a rear wall 12, side walls 14 and a diffuser disk 16 which together enclose a space. Mounted on the inner surface of the rear wall 12 are fluorescent tubes 18 which serve as light sources for the film viewing cabinet 10. The diffuser disk 16 includes a disk of transparent milky white plexiglass as described above so that light from the source can pass through the diffuser disk and is scattered. An X-ray or radiographic film 20 is secured to the diffuser disk 16 by clips or other suitable means (not shown) so that light passing through the diffuser 16 illuminates an X-ray image 22 located on the film 20. The light transmits an image of the X-ray film image 22 to the retina 24 of the eye of an observer.

Fig. 2A shows a portion of an X-ray film 20 which is drawn at a distance from a portion of the diffuser disk 16 for the purpose of illustration, although in reality they are together as shown in Fig. 1. The portion of the diffuser disk 16 contains light from fluorescent tubes 18 and directs scattered or diffused light rays 26 to the X-ray film 20. A portion of the X-ray image 22, which contains lighter and darker areas 22a and 22b, respectively, is projected by the light rays 26 onto the retina 24 in the form of lighter and darker images 28a and 28b, respectively, which correspond to the areas 22a and 22b.

Furthermore, Fig. 2A shows that a portion of the scattered light rays 26, designated 26a, pass through the film at a boundary or edge 30 between lighter and darker image areas 22a and 22b and project an image 32 of the edge 30 onto the retina 24. However, due to the diffuse nature of the light rays 26a, the image 32 is a blurred representation of the edge 30, not a clear and sharp image. As a result, the resolution between the parts 20a and 20b of the x-ray image is degraded as stated above. It thus becomes more difficult to accurately interpret the medical information provided by the x-ray image 22.

In Fig. 2B, an x-ray image 20 is shown which is traversed by light rays 34 whose solid angle is substantially smaller than the solid angle of the light rays 26. In one case, the solid angle could be on the order of 0° so that all the rays 34 are parallel to each other. In this case, all the light rays 34 intersect the plane of the film 20 at the same angle, such as 90°, rather than at different angles varying over a wide range. The light rays 34 project the regions 22a and 22b onto the retina 24 as respective images 35a and 35b in the same manner as the images 28a and 28b described above. A portion of the light rays 34 which pass through an edge 30 on the image 20 and are shown in Fig. 2B as light ray 34a also projects a sharp and unsmeared image 36 of the edge 30 onto the retina 24. A The viewer is thus able to distinguish much more clearly between light and dark areas 22a and 22b, and this represents a significant improvement in resolution and contrast.

Fig. 3 shows a film viewing box 38 which generally comprises a light source 40, a rear wall 42 and side walls 44, each of which is similar or identical to the fluorescent tubes 18 and walls 12 and 14 described above. The diffuser disk 46 is similar or identical to the diffuser 16 and is arranged to diffuse light passing through it from the light source 40 to the front of the film viewing box 38, i.e., to the right as viewed in Fig. 3. Fig. 3 also shows a disk 48 in front of the film viewing box 38 which is conveniently made of tempered flat glass having a thickness on the order of 0.16 mm (1/16 inch). The X-ray film 20 described above is secured to the glass disk 48 by suitable means (not shown).

Parallel light rays 34, which have a solid angle of 0º, generally provide the highest resolution and contrast when illuminating a film. However, as described below, in some applications there are certain advantages when the illuminating light is not parallel, but has a solid angle that is significantly smaller than the solid angle characteristic of the light rays 26.

In accordance with the invention, Fig. 3 further shows a sheet or film of light directing material 50 disposed over the front end of the viewing box 38 between the diffuser 46 and the X-ray film 20, as will be described below. Preferably, the film 50 is disposed against the disk 48 so that all the light passing through the diffuser 46 and scattered is incident on the foil 50, which serves to improve the resolution and contrast when viewing the image 22 on the X-ray film, as will be explained.

The film 50 is selected to be made of a film material which directs light incident at a critical angle of θc with respect to the plane of the film 50. More specifically, when light rays from the diffuser 26 impinge on the film 50 at angles less than θc, then at least a portion of the light rays are reflected by the film 50 back into the interior of the film viewing box 38 and do not pass through the glass plate 48 to illuminate the x-ray film 20. However, when other light rays from the diffuser 46 impinge on the film 50 at angles greater than θc, then at least a portion of such light rays pass through the light directing film 50 but are refracted thereby into light paths which are at a substantially smaller angle with respect to an axis perpendicular to the plane of the film than they were prior to refraction. Thus, Fig. 3 shows incident light rays 52a and 52b reflected from the film 50 because their respective incident angles θ1 and θ3 are smaller than the critical angle θc. On the other hand, the light rays 52c, 52d and 52e are each refracted by the film 50 as described above because their respective angles θ5, θ4 and θ2 are all larger than θc. Therefore, they are redirected or converged to lie at a smaller solid angle before passing through the plate 48 to transilluminate the x-ray film image 20 and transmit it to the retina 24. The resolution and contrast of the image received by the retina is therefore significantly improved over an image provided by light with a significant scattering component. Since the light-directing foil 50 and the X-ray film 20 rest against parallel opposite surfaces of the glass plate 48, the respective planes 50 and 20 lie parallel to one another.

It is understood that the degree of convergence of the refracted light rays 52c-e depends on the material selected for the film 50. Thus, in one application, the light rays 52c-e can be converged such that their respective refracted paths lie at a solid angle on the order of 0º. In this case, the light rays 52c-e run parallel to one another and perpendicular to the plane of the film 20. In another application, the light rays 52c-e can converge such that their associated solid angle is on the order of 90º.

It will be further understood that varying amounts of scattered light, i.e. light not limited by the solid angle of the reflected light, will pass through the film 50, depending on the material selected for it. Contrast and resolution of an illuminated image film 20 is generally improved by reducing the scattered light component of the light illuminating the film and by reducing the solid angle of the reflected light component of the illuminating light.

Fig. 3 shows a diffuser disc 26 at a distance from the light-directing film 50, specifically to the left of the latter. In an alternative embodiment, the diffuser disc 46 could also be in contact with the film 50. It is also clear that the film 50 can be used to reduce the light emitted by a given Light source 40 is effective in illuminating the X-ray film 20, since part of the light is reflected back into the film viewing box as mentioned above. It may therefore be necessary to increase the brightness of the light source, for example by adding more fluorescent tubes or a reflector.

It has already been stated that various film or sheet materials can be used for the light directing sheet 50 to produce the above-mentioned results. However, it has been found that a particularly suitable material for this purpose is a product of the Minnesota Mining and Manufacturing Company called Brightness Enhancement Film. Figures 4 and 5 together show a sheet 54 of such material which has a smooth side 56 while a large number of parallel microscopic ribs 58 are formed in the opposite side. Each rib 58 extends across the surface of the sheet or sheet 54 and has a triangular cross-section 59 of microscopic size. Thus, each rib 58 forms a microprism. Such microprism films 54 have a thickness of the order of 0.1 mm and a distance between the tips of adjacent ribs of the order of 0.1 mm. Hypothetical reference axes such as axis A shown in Fig. 4 are used here in connection with microfilm sheets 54 to indicate the respective orientations of their parallel ribs 58. The ribs or microprisms 58 of the sheet 54 are very effective in directing incident light rays, as stated above, which lie in planes perpendicular to the rib direction or have large components in that direction. However, components of scattered light lying in other planes tend to pass through the sheet 54 without changing their direction. Thus, when the microprism sheet 54 is used as a light directing film, then two sheets of film 54 in the manner shown in Fig. 6 are more effective than a single sheet in reducing stray light components of the light illuminating the X-ray film 20. Fig. 6 thus shows a film viewing box 60 which is modified from the film viewing box 38 described above in such a way that the single film 50 is replaced by two sheets of microprism film 54. The two film sheets, which are respectively referred to in Fig. 6 as microprism films 62 and 64, are oriented with their microprism ribs parallel to the axes Ax and Ay. The films 62 and 64 lie parallel to one another and to the X-ray film 20 and are arranged between the diffuser disk 46 and the glass plate 48 on which the X-ray film 20 rests in the manner described above. The smooth sides 56 of the microprism sheets 62 and 64 are directed toward the light source 40 and the diffuser 46 of the film viewing box 60, as best shown in Figs. 7 and 8. Their microprism ribs 58 face the X-ray film 20.

For illustrative purposes, the film viewing box 60 in Fig. 6 is oriented along the Z axis of a rectangular coordinate system with the axes X, Y and Z at right angles to one another. The films 62 and 64 are both parallel in the XY plane and perpendicular to the Z axis. The reference axes Ax and Ay are parallel to the X and Y axes, respectively, so that the microprism ribs 58 of the film 62 are oriented at 90º to the ribs of the film 64. The film 62 thus acts on light rays that lie in the XZ plane or have significant portions in it, such as the light rays 66a and 66b according to Fig. 7. The film 62 reflects light rays 66a back into the film viewing box 60 because their angle of incidence is smaller than the critical angle θc is.

On the other hand, the angle of incidence of the light rays 66b is greater than the critical angle θc, so that the light beam 66b is refracted by the foil 62 in a beam path with a smaller angle relative to an axis running perpendicular to the plane of the foil 62 and towards the plane of the X-ray film 20.

The film 64 has a similar effect on light rays that lie in the Y-Z plane or have significant portions in it, such as the light rays 68a and 68b in Fig. 6. The film 64 reflects the light beam 68a back into the film viewing box 60 and refracts the light beam 68b into a beam path that is at a smaller angle to the axis running perpendicular to the plane of the film 64 and thus to the X-ray film 20.

Fig. 9 shows a film viewing box 70 which is modified from the film viewing box 38 in that the single sheet 50 is replaced by four sheets or layers of microprism sheet 54, each of which is designated as film layers 72a-d. As with the sheets 62 and 64 of the film viewing box 60, the sheet layers 72a-d are laid together and disposed between the diffuser disk 46 and the glass plate 48 which supports the X-ray film 20, parallel to each other and to the film 20. The respective sheet layers 72a-d are parallel to the XY plane of a rectangular coordinate system and perpendicular to the Z axis thereof. Furthermore, the sheet layers 72a-d are arranged such that their respective microprism ribs 58 are oriented or aligned at angles of 45° to each other. This is shown in Fig. 9, where the axis A1 corresponding to the film layer 72a forms an angle of 0º with respect to the X-axis. The respective film layers 72b-d The corresponding A₂ to A₄ are oriented around the Z-axis at 45º, 90º and 135º to the X-axis, respectively.

The use of four microprism film layers in the arrangement described above has been found to be particularly effective in further reducing stray light components that can pass through them to the x-ray film 20. However, it has been found that the total amount of light passing through the microprism film layers 72a-d to the x-ray film 20 can be significantly reduced for a light source 40 of a given intensity. Such a loss of brightness can be easily compensated for by additional fluorescent tubes or by adding a reflector behind the fluorescent tubes, as noted above.

If all of the light rays passing through the film 20 were exactly parallel to each other, that is, if their solid angle was 0º and there was no stray light component, then only a small portion of the film would be seen. Another small portion of the same film or of another film being viewed simultaneously could be seen by the viewer only by moving his head. This would be impractical because, typically, to make a diagnosis, one would want to see the entire film as well as another comparison film. For example, the American College of Radiology specifies that old mammogram images must be compared with new mammogram images to determine changes that may be indicative of cancer. A user must be able to view a particular view of a breast on both old and new mammograms simultaneously. Thus, while it is desirable to make the light illuminating the film substantially parallel to improve contrast and resolution, it is often or It is usually desirable for the illuminating light rays to have a certain divergence, i.e. a solid angle of more than 0º. In the two embodiments described with reference to Figs. 6 and 9, the light illuminating the film achieves the desired degree of divergence in order to improve contrast and resolution and at the same time allow a sufficiently large area of the film to be viewed.

In Fig. 10A, a solid angle of 55 is shown as a characteristic of light projected from the diffuser 16 of a known film viewing box 10 to the film 20. The solid angle 55 is oriented along an axis 57 which is perpendicular to the plane of the film 20 and has a size Ca. Ca is extremely large, approximately on the order of magnitude close to 180º. The leg of the solid angle 55 is defined by the strongly scattered light rays 55a and the center of their vertex 55b lies at a point on the edge of the diffuser 16.

Fig. 10B shows a solid angle 61 as a characteristic of the light projected to illuminate the film 20 which has been directed back through the film 50, such as the microprism film 50. The solid angle 61 is oriented along an axis 63 which is perpendicular to the plane of the film 20 and has a size C'a which is conveniently on the order of 90º. The leg of the solid angle 61 is defined by light rays 61a which are much narrower than the light rays 55a and their vertex 61b lies at a point on the edge of the film 50.

Fig. 11 shows an arrangement which is used for the quantitative contrast comparison of picture films which are each illuminated by an arrangement according to Fig. 9 or the known film viewing box. Fig. 11 shows a conventional film viewing box 10 which was described above in connection with Fig. 1 described above except that the light source 12 is replaced by a strobe light (not shown) over which is drawn a thin fabric (also not shown) which acts as an additional light scattering means. The combined action of the strobe light, the fabric and the diffuser disk 16 serves to provide extremely uniform illumination of an X-ray or other photographic film disposed on the diffuser disk 16.

Fig. 11 further shows two identical grayscale wedges 74 and 76, to be described later, which are attached to the diffuser disk 16 by suitable means not shown. The step wedge 74 is attached directly to the diffuser 16. A portion of the film layers 72a-d, which were described above in connection with Fig. 9 and are shown in Fig. 11 as film layers 78, are arranged between the diffuser disk 16 and the step wedge 76. Thus, the step wedge 74 is illuminated as in the prior art, while the step wedge 76 is illuminated by the embodiment of the invention illustrated in Fig. 9.

Each of the step wedges 74 and 76 is a product known in the art which is produced from a piece of photographic film by successively exposing small adjacent rectangular areas to increasingly stronger light values. This produces a series of small rectangles or steps 80 and 82, respectively, each slightly darker than the previously exposed step. A typical step wedge contains twenty-one consecutively numbered steps, the first of which is transparent and the twenty-first or last dark. Wedges 74 and 76 are commercially available products which can be used with comparatively high precision for calibration. and similar purposes and are selected to be identical.

Fig. 12 shows a photograph 84 of gray level wedges 74 and 76 obtained while the wedges were each illuminated in the manner described above in connection with Fig. 11. The photograph 84 thus represents images 74a and 76a which are respectively images of the step wedges 74 and 76 that the eye of a viewer (not shown) would see at the location of a camera (not shown) used to produce the photograph 84. The images 74A and 76A comprise images 80A and 82A of the corresponding steps 80 and 82, respectively. As is known to those skilled in the art, the contrast between two adjacent areas of a film is defined as the difference between the respective brightness or light transmission through the two areas divided by the average transmission of both. If 71% and T2 are the respective transmissions through the two areas and Cl12 is the contrast between them, this relationship can be represented mathematically as:

Equation (1) was used to quantitatively demonstrate the contrast enhancement between adjacent step images 82A of image 76A associated with film layers 78 of the invention as compared to prior art step images 80A. This was done by first measuring the optical density for each of the step images 82A and also for each of the step images 80A corresponding to grayscale wedge levels 7 through 12. The gray shades of these levels are radiologically useful. (The gray levels of steps 1 through 6 are generally too light for diagnostic purposes and the shades 30 to 31 were too dark.) The optical density was determined by sequentially projecting light from a light source 86 through the respective step images 80A and 82A and measuring the corresponding optical density values with a densitometer 88, such as the Model TBX manufactured by Tobias and Associates, Inc. Table I shows the respective optical density measurements corresponding to gray scale levels 7 to 12 for both images 74A and 76A. TABLE I

As is known, the transmittance T through a region is related to the optical density O.D. via the following equation:

O.D. = 1/log T Equation (2).

It is understood that the logarithmic term in equation (2) refers to the logarithm to base 10. Applying equation (2) to corresponding values of optical densities as shown in Table I, the transmittance for each of levels 7 through 13 of each of images 76A and 74A can be readily determined. The respective transmittance values are shown in Table II. TABLE II

From equation (1) and the transmittance values of Table II, the contrast between steps 7 and 8, 8 and 9, 9 and 10, 10 and 11, and 11 and 12 for both images 74A and 76A can be readily calculated. These respective contrast values are listed in Table III, which shows the percent contrast improvement between pairs of adjacent step images of image 76A compared to the corresponding step image pairs of image 74A. By averaging the percent improvement values of Table III, it can be shown that the invention improves contrast over a prior art film viewing box by an average of 37% for the five step wedge pairs listed in Table III. TABLE III

To compare the improvement achieved by the invention over a prior art film viewer, five radiologists, each holding certifications from the American Board of Radiology and diplomas in mammography from the American College of Radiology, and five registered radiologic technicians, each holding mammography diplomas from the American College of Radiology, were asked to view a film of the standard American College of Radiology mammography phantom illuminated with a prior art film viewer and with a film viewer according to the embodiment of the invention illustrated in Fig. 6. Each of the radiologists and technicians found that the resolution with this embodiment was significantly superior to the resolution with the prior art film viewer.

Obviously, many modifications and variations of the invention are possible in light of the above teachings. For example, one modification is that the proportion of parallel light contained in the total amount of light exposed to the film could be increased only by refracting the light rays at angles of incidence exceeding the above-mentioned critical angle. In another modification, the same result could be achieved only by reflection.

Claims (9)

1. Film viewing box for illuminating and viewing a medical diagnostic film (20) with - a housing having rear and side walls (42, 44), - a holding device (48) for holding the film (20) in a certain plane on the front side of the housing, - a light source assembly (40) and a diffuser assembly (46) arranged to diffuse light passing through it from the light source, the assemblies being located within the housing to project light to the film (20) held by the holding device, the projected light having a first solid angle, - a light-directing film arrangement (50) which is arranged within the housing between the diffuser arrangement (46) and the holding device (48) in such a way that it redirects a part of the light projected to illuminate the film (20) so that this redirected illumination light has a second solid angle which is substantially smaller than the first solid angle, where the light directing film assembly (50) comprises a selected number of layers of microprism films (54) and microscopic prism elements (58) are formed in each layer which form a plurality of parallel microscopic ribs and selectively refract and reflect portions of the projected light to form the redirected illumination light. 2. Device according to claim 1, wherein the first solid angle is of the order of 180° and the second solid angle is of the order of 90°. 3. Device according to claim 1, in which - a critical angle is assigned to the light-directing foil arrangement (50), - the projected light contains light rays which extend at angles to the specific plane which are each smaller than the critical angle and which comprise a first part of the projected light, the projected light further containing light rays which extend at angles to the specific plane which are each larger than the critical angle and which comprise a second part of the projected light, and - the microscopic prism elements - prevent the first part of the projected light from being transmitted to the film (20), - and refract the second part of the projected light and converge it so that it lies within the second solid angle. 4. Device according to claim 1, characterized in that the light directing film arrangement (50) contains a plurality of layers (62, 64) of the microprism film. 5. Apparatus according to claim 4, wherein the apparatus includes means (46, 48) for supporting each of the film layers parallel to the particular plane. 6. Device according to claim 5, wherein each of the microprism film layers (62, 64) comprises a number of parallel rows of microprism elements (58) and the rows of one of the layers do not run parallel to the rows of another of the layers. 7. The device of claim 6, wherein the device includes a first and a second layer (62 and 64, respectively) of the microprism film (54), and the rows of the first and second microprism film layers are oriented perpendicular to each other. 8. The device of claim 6, wherein the device includes four adjacent layers (72a-d) of the microprism film (54) and the rows of adjacent film layers are at 45° to each other. 9. Device according to claim 1, wherein the light source arrangement and the diffuser arrangement (40,46) comprise a projection light source and a diffuser disk (46) which are arranged between the light source (40) and layers of microprism films (54).